Reduction in situ of a dipolar molecular gas adhering to a substrate



GAS ADHERING TO A SUBSTRATE 3 Sheets-Sheet 1 Filed Jan. 29, 1962 Oct. 13, 1964 E. MATOVI CH REDUCT IN SITU A DIPOLAR MOLECULAR A ADHERIN O A SUBSTRATE Filed Jan. 29, 1962 3 Sheets-Sheet 2 lira-a.

p -Qfixa 5/ flaw/oz, 0 Jam/1147014644 '7 Oct-l3, 1964 E. mA 'rovlcH I REDUCTION IN SITU OF A DIPOLAR MOLECULAR GAS ADHERING TO A SUBSTRATE Filed Jan. 29, 1962 3 Sheets-Sheet 3 reaction,

United States l atent C) 3 152 as: REnUcrroN IN sun or A nn'nrxn MOLECU- ran GAS ADHERHNG TO A sunsraaru V EdwinMatovich, Costa Mesa, Calif 'assignor to Hughes- This invention relates to deposition of crystal mate rial on substrates, and is particularly directed to nonepitaxial growth, or deposit, of semiconductor crystal material as abase for subsequent epitaxial growth.

Epitaxial growth of crystal material is the growth of a new crystal material upon a base in a manner to duplicate andextendthe crystal system of the base. Single crystal material may be produced upon single crystal base material, and poly-crystalline material'may be grown on polycrystalline base materials. Epitaxial deposit of crystalline material has been much used insemiconductor device manufacture to deposit high resistivity material upon low resistivity material for subsequent processing in the high resistivity region. Mechanisms for epitaxial deposit include disproportionation, as

exampled by thereaction,

2Gel =Ge+GeI 1 used to deposit germanium epitaxially on germanium seed crystals; thermal decomposition as exemplified by the six,=si+2x where halogen such as iodine or chlorine, used to epitaxially deposit silicon on silicon base crystals; and reduction, exampled by the reaction zu +stcn=si+4nci (3) methods ofepitaxial crystal growth,

These and other sometimes called vapor growth, require seed crystals, or

as quartz, oxides, ceramics and metals. A reducible dipole molecular gas of the material to be coated is first passed over the desired surface at reaction temperatureto coat it with a suitable adherent film, and the film is then reduced (or converted) to the material to be grown, and oneof the processes for epitaxial growth is commenced. For a further consideration of what I believe to be novel and my invention, attention isdirected to the balance of this specification, the claims, and the drawings in which: FIGS; 1, 2'and3 are respectively a diagram of equipment'use'd 'andii'ow diagrams of processes according tothis invention. 7

FIG. 4' is a schematic sketch ofchemical reactiorisQ l andidipole arrangements at a material surface which is if. proposed to explain the process'illustratedin FIGS. 1-3.

P16. '1 schematically illustrates equipment used for growing single crystal germanium on substrates ofquartz, anodized metals. and alumina ceramlcs. A flowing gas system, is illustrated containing, in success on, a gas purication train, a metering and evaporation section, a mixing, and reaction section, and termination.

- A source of pure hydrogen gas isprovided .by delivery of hydrogen gas through a valve 11 in pipe 12 and through 'acatalytic reactor 13 to react'any oxygen present to wathen through a liquid nitrogen cold trap 19 to remove residual water vapor and oxygen. The H gas is next metered through'valve 15 to bubbler 16 where it passes through liquid germanium tetrachloride (GeCl the vapor, of which is entrained in a volume proportional to the vapor pressure of liquid, and the GeCL, rich H gas is mixed in mixer 17 with pure carrier H gas from valve 18 and passed through a gas mixing chamber 20 into a furnace or reaction chamber 21. Mixer 17 and mixing chamber 20 are designed to avoid stratification of light H gas from the heavier gases. A quartz boat 22 carriessubstrates 23 upon which germanium is tobe deposited, at least one surface thereof being preferablysubstantially masked at' vents through an oil bubbler 25 to prevent back diifusion of atmospheric air into the system, and the exhaust from bubbler 25 is a flare burner 26.

A source 27 of argon (A) gas for dilution or purging is connected ahead of'the desiccant chamber 14 and sources of other gases for doping the growing deposit of crystal material are provided in bubblers 31 and 32 which are connected to the hydrogen gas train in parallel with the germanium v tetrachloride bubbler. Phosphorus trichlo-ride is supplied through bubbler 31 subject to control of valve 34', and boron tribromide is supplied through bubbler 32 subject'to control of valve 35. After epitaxial growth has started, the'conductivity type of the growth and its doping level may be controlled by alternate flow of enriching boron tribromide and phosphorous trichloride, to produce in turn P and N-type layers of germanium crystal material. Any of the haldies may be used so long as suitable temperatures are maintained for vapor pressure control.

To grow germanium on a desired substrate surface, such as oxidized metal (including oxidized germanium) or quartz, the substrate 23 here taken to be anodized metal (such as tantalum or quartz, is first placed in the chamber 21, and the reaction train is then purged with purified hydrogen gas. The sample 23 in chamber 21 is then heated to between 420 C. and 590 C., and the valve 15 is opened to'entrain GeCli, fromthe bubbler 16. At tem peratures'below 420, no reaction takes place, and at temperatures above 600 -C., everything will be coated,

HGeClg is a polar, or dipole; molecule, the negative pole beingthe Cl-rich end'. Since any solid heated in the flowing gas system becomes positively charged by thermi'onic emission and adsorbs H gas, and the H side of the HGeCl molecule is positive, the HGeCl molecule attaches to the solid by electrostatic attractions. These surface molecules, deposited between 420 Clandf 590 C., aare o'riented" because of their dipole structure. The at tachrnent and orientation of the HGeCl molecule to the material surface after reaction (4) is illustratedby FIG. 4(a), the hydrogen atom being presumably aligned with an oxygen atom of the surface. The temperature may now be r'a'isedto over 600 C. in flowing H or about, 6509 C., to 700 C., at which temperatures the surface. will be hydrogen-reduced to germanium without disorienting. This hydrogen reductionmay be a two step process by-which the aligned hydrogen and oxygen atoms of FIG. 4(a) are first removed in a water-forming'reaction to associate the germanium directly onto the surface Patented Get. 1'3, 1964- the reduced Ge film.

a? as shown in FIG. 4(1)), then by HCl formation the germanium is further reduced to the form shown in FIG. 4(0). The temperature is then reduced to 420 C..to 590 C., and the germanium tetrachloride turned on again to begin epitaxial growth on the first-deposited Ge surface as illustrated in FIG. 4(d). GeCL; concentrations of less than 0.1 mole percent in H should be used to avoid an etching reaction. Growth rates of 12 microns per hour may be obtained with total gas flow of 250 cm. per minute in laboratory furnace chambers. The reduction step may also be done below 600 C. in reducing hydrogen gas.

- Selective growth of germanium on desired areas only, with poisoning of other areas, is attained by first exposing the surface to be coated to the GeCL; gas in hydrogen, as above described, at between 420 C. and 590 C., then cooling and oxidizing the surface to GeO Areas not to be coated are then cleaned of the Ge film by etchingwith HF. The coated surfaces still containing the GeO coating are next reduced, as by heating to about 600 C. or over in dry H gas to reduce the 6e0 to Ge, then cooled to 420 C. to 590 C. and subjected to dilute GeCL; as before described to produce epitaxial growth on The other, cleaned surfaces will at this time be coated with the one molecular layer of HGeCl but this will effectively poison the surface against further growth below 600 C.

In the deposit of other materials, such as silicon, the initial coating of the surface with the dipolar molecules may take place in a different temperature range, but it will be below the complete reduction temperature for reduction to pure metal in hydrogen gas, so that the process steps are similar in first coating the surface with the dipolar, metal-containing molecule below the reduction temperature, then heating in pure hydrogen gas to complete the reduction of the molecule, followed next by further metal deposit by one of the epitaxial growth processes. The word metal, as used herein, includes semiconductor materials.

For a second example, in apparatus of FIG. 1 a mixture of SiCl and hydrogen gas, suitably dried and deoxidized, may be delivered to the chamber 21 at a substrate temperature between 730 and 910 C. to coat the substrate surface with the dipolar molecules SiHCl Which may then be reduced to a silicon coating by very reducing hydrogen gas or by heating in hydrogen gas to over 910 C.

While the chloride gases are preferred for formation of the polar molecules, and are sufficiently polar to produce highly oriented films and large areas of single crystal semiconductor material upon completion of the epitaxial growing process, other halogens may be used, namely bromides and iodides, and the reaction temperatures may be adjusted accordingly.

When the above discussed tetrahalide gases are exposed to semiconductor materials in hydrogen gases, the hydrogen trichloride compound formed enters into a reversible reaction, and unless very reducing conditions are maintained, an etching condition is obtained. This is avoided by'maintaining very dilute, less than 0.1 mole percent, mixtures-in the case of'GeCL; and its product dipole gas GeHCl Similar dilute mixtures are required for other compounds to maintain the reduction reaction rate ahead of the etching reaction rate. 7

The process ofcoating a substrate with an adherent molecular film of a semiconductor-material-containing dipole molecule, which can then be reduced without disturbing its orientation and order pattern, may be used to produce epitaxial films for semiconductor device manufacture.

As illustrated in FIG.'2, a quartz substrate 41, shown in FIG. 2a, is coated with an adherent film 42 of GeHCl shown in FIG. 2b, by the process heretofore described.

in the reversible reaction (4). The film 42 is next reduced to a germanium film 43, shown in FIG. 20, in dry, deoxidized hydrogen, preferably at 650 C. to accelerate the reaction. The quartz surface is then exposed to a stream of germanium tetrachloride in hydrogen as before, together with a small percentage of boron tribromide in about .001 mole percent, at 500 C., to deposit P-type conductivity germanium on the germanium coated quartz surface epitaxially as shown in FIG. 2d. The flow of boron tribromide is interrupted and a stream of phosphorus trichloride is substituted, growing an additional epitaxial layer 44 of N-type germanium. The resulting product, as shown in FIG. 2c, is quartz substrate having successive layers thereon of P and N-type germanium epitaxial material forming therebetween a P N junction. Suitable control of the impurity gases, BBr and PCl to getherwith temperatures, may be utilized to produce a wide range of impurity concentrations, hence crystal conductivity. It has been found that extremely pure hydrogen gas is required to maintain reducing conditions and to reduce the etching reaction. Since the reaction involved is reversible, the etching reaction tends to dissolve the surface, including the dopant impurity material, which is in turn redeposited. By reducing the etching portion of the reaction, sharper changes in resistivity in the epitaxial material are obtainable. Other dopant source compounds may be used, as for example AsCl or AsBr for P-type dopants.

FIG. 3 illustrates the production of a device utilizing the intermediate oxidation step discussed in the process in connection with FIG. 1. Selective area growth of epitaxial material is thus obtained.

FIG. 3a shows an anodizedmetal substrate 50 such as anodized molybdenum or tantalum, the surface film 51 being an electrically insulating oxide film. A polar gas film 52 in FIG. 3b is produced by exposure to SiCL; gas in hydrogen at about 800 C., or between about 730 and 910 C. The polar gas of film 52 is next reduced to silicon in a hydrogen stream at 950 C., producing a sili con film 49 on the anodized surface film 51 as shown in FIG. 30. This silicon film or coating, is believed to be monomolecular. The surface film 49 is then oxidized, as by exposure to water vapor, or oxygen, in a hydrogen gas stream to produce a silicon oxide film 53 (believed to .be largely SiO on film 51, as shown in FIG. 3d. A mask 54- against a silicon oxide etchant is next placed on the film 53 where subsequent epitaxial growth is desired, and the exposed portion of the SiO film is removed. An HF etch is preferred, and polymerizable photosensitive materials, such as Kodak Photo Resist, of Eastman Kodak Company, may be used to produce the mask 54. FIG. 3e shows the masked film 53 prior to etching, and FIG. 3 1 shows the etched film with the mask 54. The mask 54 is then removed as shown in FIG. 3g. The remaining SiO film 53 isexposed to reducing H gas to convert the same to a silicon film 55 as shown in FIG. 3h. The surface of the substrate is then again exposed to the mixture of reducing hydrogen and SiCL, to deposit an adherent film 56 of SiHCl over theanodized oxide film 51, and to epitaxially grow additional silicon over the reduced silicon film 55 while inhibiting further growth over the balance of the surface. During growth of the film 55, a

Germanium tetrachloride in a dry, deoxidizedhydrogen I stream in a concentration of about 0.1 mole'percent is passed over the substrate at 500 C., producing Gel- C1 P-type impurity producing material such as BBr is added to the silicon haloform gas in an amount of about 0.001

mole percent of the gas, or about 1% ofth'e silicon tetra etched to remove the surface, of N-type portion. A PN junction containing semiconductor crystal remains upon removal of the mask 63, as shown in FIG. 3 j, and suitable metal connectors 65 and 66 may be deposited connecting to the respective P and N-type film portions as shown in FIG. 3k. Since the film 55 is deposited on the electrically insulating layer or film 51, the resulting device is a diode. Similar techniques may be used toproduce transistors and other devices" upon noncrystalline sub strates by the process herein disclosed.

I claim:

1'. A method of growing a crystalline layer on a refractory material substrate surface, which method comprises:

exposing the surface to a reducible dipolar molecular gas of the material to'be' grown at a temperature, below the decomposition temperature of the dipolar gas, at which the dipolar molecules of the gas ad here to' said surface; and

reducing said dipolar molecules to form on the surface a layer of said material.

2. A method of growing a crystalline layer on a substrate surface of material of the class consisting of metals, quantz, ceramics, oxides and oxidized metals, which method comprises:

exposing the surface to a hydrogen reducible dipolar molecular gas of the material to be grown at a temperature, below the decomposition temperature of the dipolar gas, at which the dipolar molecules of the gas adhere to said surface; and

heating the coated surface to a temperature at which the dipolar molecules thereon are subject to hydrogen reduction, in the presence of hydrogen gas to.

fractory material substrate surface, which method comprises:

exposing the surface to a haloform gas of the material to be grown at a temperature, below the decomposition temperature of the gas, at which the molecules thereof adhere to said surface; and heating said surface to a temperature at which the adhered gas molecules are subject to hydrogen reduction, in the presence of hydrogen gas, to form on the surface a layer of said material.

4. A method of growing a crystalline layer on a refractory material substrate surface, which method comprises:

exposing the surface to a haloform gas of the material to be grown at a temperature, below the decomposi tion temperature of the gas, at which the molecules thereof adhere to said surface; heating the coated surface to a temperature at which the adhered gas molecules are subject to hydrogen reduction, in the presence of hydrogen gas to form on the surface a layer of said material; and subjecting said surface to a vapor growth process to addadditional layers of said material to said surface. .5. A method of growing a germanium semiconductor layer on a surface of a refractory substrate, which com- 6. A" method of growing a germanium semiconductor layer on a surface of a refractory substrate, which comprises:

exposing the surface to an effective mixture of germaniunr tetrahali'de' and hydrogen at a temperature be tween 420 and 590 C. to coat the surface with a film of reducible germanium compound; exposing the coated surface to hydrogen-containing reducing gas at a temperature sufiicient to reduce the germanium surface compound on the coated surface to metallic germanium; subjecting the germanium coated surface to additional decomposable" germanium compound in a" hydrogen carrier gas stream and at a temperature to deposit additional germanium on said surface.

7. A method of growing a crystalline layer on a refractory material substrate surface which method comprises:

exposin'g'the surface to a haloform gas of the material to be grown at a temperature, below the decomposition temperature of the gas, at which the molecules thereof adhere to said surface; and

heating said surface, to a temperature at which the adhered gas molecules are subject to hydrogen reduction, in the presence of hydrogen'gas, to form on the surface a layer of said material.

8. A method of growing a crystalline layer on a refractory material substrate surface which method comprises:

exposing the surface to a haloform gas of the material to .be grown at a temperature, below the decomposition temperature of the gas, at which the molecules on a surface of a refractory substrate, which comprises: exposing the surface to an effective mixture of silicon tetrahalide and hydrogen at a temperature between 730 C. and 910 C. to coat the surface with a film of reduciblesilicon compound; exposing the coated surface to hydrogen-containing reducing gas at a temperature sufficient to reduce the silicon surface compound to metallic silicon; and subjecting the silicon coated surface to additional reducible silicon compound in a hydrogen carrier gas stream and at a temperature to deposit additional silicon on said surface. 10. A method of growing a semiconductor layer of material of the class consisting of germanium and silicon on a surface of a refractory substrate, which comprises: exposing the surface to a mixture of a haloform gas of said classwith a reducible compound of said material 11'. A method of growinga layer of crystal material on a surface of the class consisting of quartz, ceramics,

oxides and oxidized metals, which comprises: exposing the surface to a mixture of a haloform gas of said material and hydrogen at a temperature suf- 'ficient to coat said surface witha reducible ,com-

pound of said material;

" exposing the coated surface to a hydrogen-containing reducing gas at a temperature sufiicient to reduce the surface compound to metal; and

subjecting the metal coated surface to an epitaxial growth process to deposit additional material on said surface.

5 12. A method of growing a layer of semiconductor material of the class consisting of silicon and germanium selectively on a refractory surface, which comprises:

exposing the surface to a haloform gas of said ma terial at a temperature sufiicient to coat said surface with a reducible compound of said material; exposing the coated surface to an oxidant to convert said surface to an oxide of said material; exposing a portion of said surface to a reducing agent to reduce said portion to said material; and subjecting the surface to an epitaxial growth process to deposit additional material on said portion of the surface. 13. A method of growing a layer of semiconductor material of the class consisting of silicon and germanium selectively on a refractory surface, which comprises:

exposing the surface to a haloform gas of said material 1 at a temperature sufficient to coat said surface with a reducible compound of said material; exposing a portion only of said surface to an oxidant to convert said surface coating to an oxide of said material; exposing the remainder of said surface to a selective reducing agent to reduce the oxidized coating portion thereof to metal; and subjecting said metal coated portion of the surface to epitaxial deposit of additional material.

References Cited in the file of this patent UNITED STATES PATENTS 2,692,839 Christensen et a1 Oct. 26, 1954 2,880,117 Hanlet Mar. 31, 1959 2,910,394 Scott et al Oct. 27, 1959 

1. A METHOD OF GROWING A CRYSTALLINE LAYER ON A REFRACTORY MATERIAL SUBSTRATE SURFCE, WHICH METHOD COMPRISES: EXPOSING THE SURFACE TO A REDUCIBLE DIPOLAR MOLECULAR GAS OF THE MATERIAL TO BE GROWN AT A TEMPERATURE, BELOW THE DECOMPOSITION TEMPERATURE OF THE DIPOLAR GAS, AT WHICH THE DIPOLAR MOLECULES OF THE GAS ADHERE TO SAID SURFACE; AND REDUCING SAID DIPOLAR MOLECULES TO FORM ON THE SURFACE A LAYER OF SAID MATERIAL. 